A remotely operable and centrally controllable locking system for motor vehicles of the kind mentioned above with which the same series of code bits each is stored in the transmitter and in the receiver, said series representing a number of sequenced code words each including a plurality of bits, is known from German Pat. No. 32 44 049. With each actuation of the transmitter, the code bits in the transmitter and receiver are stepped ahead by a constant number of bit positions corresponding to the length of a code word. When the last word is reached, switch-back is effected to the first word. Following each actuation, a check is made to see whether there is agreement between the code word transmitted and the actual code word which is up in the receiver. If there is agreement, the door is opened. Synchronization between the transmitter and the receiver is an absolute must with this system. If the synchronization should have got lost, such as by actuating the transmitter outside of the range of the receiver (so-called dummy operation), agreement can no longer be found. Thus the known system provides for reestablishing synchronization between the transmitter and receiver to a given word if a special key is pressed.
It is a disadvantage of this system that the demand for storage room in the transmitter and receiver depends directby on the number of possible combinations. It is useful, for reasons of security, to provide the greatest possible number of code words so as to obtain a very long cycle of repetition of the code words. Otherwise unauthorized "bugging" of the code might make it too easy to crack the code. What is particularly critical in respect of the security against interception is the synchronization command. If an unauthorized person finds out the code of the synchronization command, all that is required to be known is the code word which is obtained as synchronization is established. It is no longer necessary to find out the entire bit sequence.
Thus the advantage in safety offered by a permanently changing code (so-called forward stepping of the code) is greatly reduced by the need for synchronization which, in the final analysis, cancels the code stepping ahead. This becomes particularly evident when looking at the limit value. If synchronization is effected with each transmission, it is seen that a variable code and synchronization are contradictory.
The principle of forward stepping of the code is known also from DE-OS No. 33 20 721. In that case additional data are transmitted together with each word emitted and those data contain information as to the code number to be selected from the supply memorized in the receiver. Again synchronization is required between the transmitter and the receiver. It is suggested as a means of increasing the security that resynchronization should be possible only in the direction toward higher code numbers, whereby any codes recorded without authorization are deprived of their value. Moreover, the receiver is to accept resynchronization only in a very limited interval of code numbers. Still synchronization data are passed along the transmission path and, therefore, may be recorded.
The difficulties of synchronization with forward stepping of codes are described also in the laid-open German patent applications DE-OS Nos. 32 34 538, 34 07 436, and 34 07 469.